18" Moonsilver IV
1st Place Craftsmanship, 1st Place Mechanical Design
Stellafane 2007
Ross Sackett's amateur telescope making
Moonsilver IV was designed as a grab-and-go portable “weeknight”
scope using the same optics as my conventional truss telescope.  It had
to be light enough to carry, compact enough to fit through doorways
fully assembled, and attractive enough to spend the daytime in the living

It uses a conventional Newtonian optical design, with 18” f/4.3 Pegasus
primary and 3.1” Astrosystems secondary.  The primary is mounted in a
6-point PLOPimized cell with polymer edge supports at plus and minus
45 degrees.

The mounting combines a conventional dobsonian azimuth bearing with
a hybrid “dob-fork” three point altitude bearing.  Weight was reduced by
using lightweight materials and sandwich construction, and simplifying
the structure to a minimum.  Most construction is of plywood and some
softwood, with mahogany veneer.  The single pole is a commercial
carbon-epoxy tube, and the pole support stock is veneered with carbon

The focuser board includes a variable iris to exclude stray light.  The
secondary is mounted on a curved spider to minimize diffraction spikes,
with an oval velvet-covered secondary baffle in the lightpath of the
primary to reduce its size (because it is seen edge-on it has minimal
impact on the image).  The mirrorbox is also decked in black velvet to
minimize stray light, as is the integral mirror cover.  The latching mirror
cover is held open by a rare earth magnet.

Fully assembled the scope weighs 63 pounds, and can be carried in the
hands with the mirror-box latched to the rocker.  The telescope is
surprisingly stiff for a single-pole design.  After a sharp rap it oscillates
freely for several seconds, but dampens immediately if you keep a hand
on the pole.  While tracking the flexure is limited to about one arcminute
(1-2 Jupiter diameters) which is fine for a visual scope.  An aperture
mask can be used to convert the telescope to a 6” f/13 unobstructed
folded Herschelian for planetary and lunar observing.

Moonsilver IV won first place prizes in craftsmanship and mechanical
design at Stellafane 2007.
I think Moonsilver IV is the apex of my single-pole
series, and it many ways it is the simplest and most
minimalist of the Moonsilver scopes despite its large
18" mirror.  It was built over a period of four weeks
following RTMC where its big blue brother took a
couple of awards.  The blue scope was a little
heavier than I felt comfortable carrying, so for
Moonsilver IV (on the left) I substantially reduced the
weight by using sandwich construction and
lightweight materials like carbon-epoxy.

The construction of Moonsilver IV is detailed in
webarticle and was the topic of an
S&T article by
Gary Seronik.
My telescopes and ATM Projects